1.Field of the Invention
The invention relates to an improved actuator unit comprising a piezoelectric actuator.
2. Description of the Prior Art
Actuator units of the type with which this invention is concerned are used, among other ways, in fuel injection systems and particularly in fuel injection valves, since the switching times of such actuator units are very fast. The short switching times allow more-exact dimensioning of the injected fuel quantity and enable improved shaping of the course of the injection over time. The overall term “fuel injection valve” is understood in conjunction with the invention to mean all types of fuel injection valves, such as injectors for common rail injection systems or injection nozzles of conventional fuel injection systems. A fuel injection valve with a piezoelectric actuator is actuated by subjecting the piezoelectric actuator to an electrical voltage, and as a result the piezoelectric actuator, because of known physical effects of the piezoceramic, expands rapidly and lifts the valve closing member from a valve seat. The piezoelectric actuator has a certain mass, which is accelerated. If the voltage applied to the actuator is reduced, the actuator has the tendency to contract. Because of the mass inertia of the previously accelerated mass of the actuator, the result, depending on the triggering speed, is tensile forces in the actuator, which cause damage to the piezoelectric actuator, especially cracks in the soldered connections between individual layers of the piezoelectric actuator. To avoid such damage, the procedure has changed to prestressing the piezoelectric actuator in the axial direction by means of a cylindrical hollow body embodied as a spring. One such arrangement is known, for instance from International Patent Disclosure WO 00/08353 (Siemens). This hollow body is bent from a flat metal sheet and is welded at the first seam thus created. The first seam extends parallel to the longitudinal axis of the hollow body. 
Welding the first seam has the following disadvantages, among others: The welding causes what as a rule is an unwanted alteration in the microstructure of the hollow body in the immediate vicinity of the weld seam. A second problem is the splashes that occur during welding, which can cause difficulties in assembling the actuator unit or even lead to functional failures of the fuel injection valve, if one or more such splashes come loose during operation. A third problem is the subsidence of the weld seam (seam drop) at the beginning and end of the weld seam, and the resultant notch effect and increase in stress.